This invention relates to a new crystalline form of calcium M phosphate where M is sodium or lithium and a method for its preparation. More particularly, this invention relates to unique asbestiform crystalline calcium M phosphate having the form of very thin, flexible fibers and to a process for preparing such asbestiform crystals, i.e., having the form or appearance of asbestos.
Each year billions of kilograms of asbestos, fiberglass and other inorganic fibers are used in a wide variety of applications, such as for composites with organic polymeric materials like plastics, resins, natural and synthetic elastomers and the like. Such composites are used to make floor tiles, gaskets, brake linings, clutch facings and numerous other objects commonly found in industry and in the home.
Although satisfactory performance is achieved by the use of such fibrous materials in these applications, they also have certain disadvantages. For example, many of these fibrous materials, especially asbestos, are suspected of causing a rare cancer, mesothelioma, as well as other diseases, such as asbestosis, bronchogenic cancer and gastrointestinal cancer (see, for example, "Asbestos-Related Disease", L. Preges et al., Grune and Stratton, New York, 1978 and "Asbestos--Properties, Applications and Hazards", Editor, L. Michaels et al., John Wiley and Sons, New York, 1979). In addition, many inorganic fibers, such as the alkali metal titanates, are suspected of causing such health problems among users of the fibers. On the other hand, some inorganic fibers are soluble in water, dilute acid or dilute alkali which limits the usefulness of such fibers.
Condensed phosphates are the only known variety of inorganic substances which are capable of forming fibers and at the same time are normally found in living systems. J. R. VanWazer reports in "Phosphorus And Its Compounds", Vol. 1, Interscience Publishers, Inc., New York (1959) at pages 456 and 800 that long chain phosphates have been identified in yeast, molds, algae and bacteria. An enzymatic synthesis of long chain phosphate salts has been described and is believed to occur by donation of the terminal PO.sub.4 group of adenosine triphosphate. Although there is still much to be learned about the various phosphatases which catalyze the schism of the P--O--P linkages, it is known that the enzymatic hydrolysis of chain and ring compounds can be extremely rapid in the presence of dipositive cations, such as calcium and especially magnesium. Hence, a fiber or asbestiform crystal which can degrade to a nutrient in the body while remaining stable in the absence of enzymes should create no health problems and should be suitable for many applications where fibrous materials are needed.
Those skilled in the art are aware that fibers can be prepared by drawing a fiber from a glassy phosphate or that certain phosphates, such as crystalline potassium metaphosphate, sometimes called potassium Kurrol's salt, form acicular crystals. However, the usefulness of such amorphous or crystalline fibers is limited because of their solubility in aqueous systems and/or low tensile strength. In addition, such fibers are frequently too large to use for numerous applications.
Thus, it can be seen that there is a need for an inorganic fibrous material that can be prepared in very small diameters and relatively short lengths and which can withstand temperatures up to about 725.degree. C. for use as insulation and as a reinforcing aid in polymeric materials, which is virtually insoluble in water, dilute acid or dilute alkali, and which does not present undue health hazards to those using the material. Now, according to the present invention, an inorganic, asbestiform crystal has been developed which can have an average diameter of less than 1 micron and which has most of the advantageous properties of other known fibrous materials within this size range. However, unlike most other inorganic fibers, the asbestiform crystal of the present invention is attacked by enzymes of biological systems which not only renders the hydrolysis products relatively harmless to mammals, such as humans, but also provides nutrients to the biological system.